Generally, fine pattern formation is carried out by the photolithography in manufacturing processes of a semiconductor device. A number of substrates called transfer masks are usually used for this fine pattern formation. The transfer mask is generally such that a fine pattern made of a metal thin film or the like is provided on a transparent glass substrate. The photolithography is used also in the manufacture of the transfer mask.
For miniaturization of a pattern of a semiconductor device, it is necessary to shorten the wavelength of an exposure light source for use in the photolithography in addition to the miniaturization of a mask pattern formed in a transfer mask. In recent years, the wavelength of an exposure light source for use in the manufacture of a semiconductor device has been shortened from KrF excimer laser (wavelength 248 nm) to ArF excimer laser (wavelength 193 nm).
As a type of transfer mask, a halftone phase shift mask is known apart from a conventional binary mask having a light-shielding film pattern made of a chromium-based material on a transparent substrate. This halftone phase shift mask is configured to have a light-semitransmissive film pattern on a transparent substrate. This light-semitransmissive film (halftone phase shift film) has a function of transmitting light therethrough with an intensity that does not substantially contribute to exposure, and further has a function of providing the light transmitted therethrough with a predetermined phase difference relative to light transmitted in air for the same distance, thereby producing a so-called phase shift effect.
Generally, in a transfer mask, an outer peripheral area of an area where a transfer pattern is formed is required to have an optical density (OD) of a predetermined value or more so that when exposure and transfer are carried out on a resist film on a semiconductor wafer using an exposure apparatus, the resist film is prevented from being affected by exposure light transmitted through the outer peripheral area. Normally, it is said that the OD is desirably 3 or more and should be at least about 2.8 at the outer peripheral area of the transfer mask. However, a light-semitransmissive film of a halftone phase shift mask has a function of transmitting exposure light therethrough at a predetermined transmittance and therefore the optical density required for the outer peripheral area of the transfer mask is difficult to achieve only with this light-semitransmissive film. In view of this, like a phase shift mask blank disclosed in JP-A-2007-033469 (Patent Document 1), a light-shielding film (light-shieldable film) is laminated on a semitransparent film with a predetermined phase shift amount and transmittance for exposure light, thereby achieving a predetermined optical density with the laminated structure of the semitransparent film and the light-shielding film.
On the other hand, there is also present a photomask blank as disclosed in JP-A-2006-215297 (Patent Document 2). A semitransparent laminated film of this photomask blank is configured in such a way that a phase advancing film having a property that the phase of exposure light transmitted through this film is advanced relative to that of exposure light transmitted in air for the same distance and a phase delaying film having a property that the phase of exposure light transmitted through this film is delayed, vice versa, are laminated together. With this structure, it is possible to prevent the occurrence of a phase difference between exposure light transmitted through the semitransparent laminated film and exposure light transmitted in air for the same distance. Even with the semitransparent laminated film having such a property, the optical density required for the outer peripheral area of the transfer mask is difficult to achieve alone. Therefore, also in the photomask blank disclosed in Patent Document 2, a light-shielding film is laminated on the semitransparent laminated film, thereby achieving a predetermined optical density with the laminated structure of the semitransparent laminated film and the light-shielding film.